1. Field of the Invention
The present invention relates to a method for manufacturing an electroluminescent (hereinafter, “electroluminescent” may be abbreviated as “EL”.) element having a light emitting layer formed by using a photolithography method.
2. Description of the Related Art
The EL element couples a positive hole and an electron injected from facing electrodes in a light emitting layer, excites the fluorescent substance in the light emitting layer by the energy, and emits light of a color corresponding to the fluorescent substance, attracts attention as a flat display element of self light emission. In particular, an organic thin film EL display using an organic substance as the light emitting material has high light emitting efficiency such as realizing light emission with a high luminance with less than 10 V coated voltage and is capable of emitting light with a simple element structure. Thus, application thereof to an inexpensive simple display, which displays a specific pattern by light emission, for advertisement and the like, is expected.
In manufacturing of a display using such EL element, patterning of a first electrode layer and an organic EL layer is usually carried out. As the method for patterning the EL element, a method of vapor depositing the light emitting material via a shadow mask, a method of coating divisionally by ink jet, a method of destroying a specific light emitting pigment by the ultraviolet ray irradiation, screen printing method, or the like can be presented. However, in these methods, an EL element which realizes all of high light emitting efficiency, high light taking out efficiency, manufacturing process simplicity and highly sophisticated pattern formation could not be provided.
As a means for solving these problems, a method for manufacturing an EL element by patterning a light emitting layer by a photolithography method has been proposed. By this method, compared with the conventionally executed patterning method by the vapor deposition, manufacturing can be executed relatively easily and inexpensively since vacuum equipment having a highly accuracy aligning mechanism or the like is not needed. In contrast, compared with the patterning method by the ink jet method, it is preferable in that a pre-process to a structure, a substrate or the like for aiding patterning is not executed. Furthermore, in relation with the discharging accuracy of an ink jet head, the manufacturing method by the photolithography method is more preferable for the highly sophisticated pattern formation, and thus it is advantageous.
As a method for forming plural light emitting parts by such photolithography method, for example, a method shown in FIGS. 3A to 3N has been proposed.
First, as shown in FIG. 3A, a first light emitting layer coating solution 2 is coated on the entire surface of a substrate 1 having a first electrode layer (not shown in the figure). Next, as shown in FIG. 3B, a positive type resist 3 is coated on the entire surface, and as shown in FIG. 3C, with only the parts, where first light emitting parts are to be formed, masked with a photomask 6, the other parts are exposed to ultraviolet ray 7.
By developing the same with a resist developing agent and cleaning with water, as shown in FIG. 3D, the resist of the exposed parts is removed so that the resist 3 remains only in the unexposed parts. Furthermore, by developing with a solvent of the light emitting layer or the like, as shown in FIG. 3E, the exposed first light emitting layer 2 is removed so that the resist 3 and the first light emitting part 2′ covered with the resist 3 remain.
Next, same as the formation of the first light emitting part 2′, as shown in FIG. 3F, a second light emitting layer coating solution 5 and a positive type resist 3′ are coated on the entire surface. At the time, as it is apparent from FIG. 3F, there is a part where the second light emitting layer coating solution 5 coated on the entire surface and the first light emitting part 2′ are in contact. That is, as mentioned above, the surface of the first light emitting part 2′ remaining on the substrate 1 is covered with the positive type resist 3, and the end part “a” developed with the light emitting layer developing agent is exposed. Therefore, when the above mentioned second light emitting layer coating solution 5 is coated on the light emitting part 2′, the first light emitting part 2′ and the second light emitting layer coating solution are contacted at this end part “a”. Thereby, there has been a problem of elution of the first light emitting part into the second light emitting layer coating solution so as to generate a trouble of color mixture and pixel narrowing.
Next, as shown in FIG. 3G, with the parts, where the first and second light emitting parts are to be formed, masked with the photomask 6, the other parts are exposed to the ultraviolet ray 7.
By developing the same with the resist developing agent and cleaning with water, as shown in FIG. 3H, the resist 3′ of the exposed parts is removed. Furthermore, by developing with the solvent of the light emitting layer, as shown in FIG. 3I, only the exposed second light emitting layer 5 is removed so that a second light emitting part 5′ covered with the resist 3 is formed.
Furthermore, same as the formation of the first and second light emitting parts, as shown in FIG. 3J, a third light emitting layer coating solution 9 and a positive type resist 3″ are coated. At the time, as it is apparent from FIG. 3J, the first light emitting part and the third light emitting layer coating solution 9 are contacted at the end part “a” of the first light emitting part 2′ formed initially, and further, the second light emitting part 5′ and the third light emitting layer coating solution 9 are contacted at the end part “b” of the second light emitting part 5′. Therefore, there is a possibility that the first light emitting part 2′ and the second light emitting part 5′ are eluted into the third light emitting layer coating solution so as to generate a trouble of color mixture and pixel narrowing.
Then, as shown in FIG. 3K, with the parts, the first, second and third light emitting parts are to be formed, masked with the photomask 6, the other parts are exposed to the ultraviolet ray 7. By developing the same with the resist developing agent and cleaning with water, as shown in FIG. 3L, the resist 3″ of the exposed part is removed. Further, by developing with the solvent of the light emitting layer, as shown in FIG. 3M, only the exposed third light emitting layer 9 is removed so that a part 9′ covered with the resist 3″ remains. Then, by coating a peeling treatment with a resist peeling solution, the part with the resist formed thereon and the upper layers thereof are peeled off so that the three color light emitting parts of the first light emitting part 2′, the second light emitting part 5′ and the third light emitting part 9′ are formed as shown in FIG. 3N. By forming a second electrode layer on these light emitting parts, an EL element emitting the EL light toward downward in the figure can be manufactured.
As mentioned above, by the photolithography method, since the end part “a” of the patterned first light emitting part and the end part “b” of the second light emitting part are not covered with the photoresist layer, there has been a problem of elution of the patterned light emitting part into the light emitting layer coating solution to be coated later at the time of coating the subsequent light emitting layer coating solution so as to generate a trouble of color mixture and pixel narrowing at the end part thereof.
Moreover, for example, as shown in FIG. 3M, since plural light emitting layers and the photoresist layers are laminated on the first light emitting layer 2′ and the second light emitting layer 5′, at the time of coating the peeling treatment to obtain only the light emitting layer, the area of the photoresist layer where contacting to the photoresist peeling solution is minute so that there is a problem that an extremely long time is required for sufficiently providing the effect of the photoresist peeling solution to the photoresist layer. Long time exposure of the substrate to the photoresist peeling solution is not only disadvantageous in terms of the manufacturing efficiency but also a problem of swelling and eluting the pattern formed layer can be generated from the influence of the photoresist peeling solution. Therefore, a method for obtaining a state in which the photoresist peeling solution can easily act at the time of coating the peeling treatment is also required.